The present invention relates to a process for the production of a nutritional product prepared from an aqueous cereal composition through extrusion cooking.
More particularly, the invention relates to the production of precooked cereals to provide a precooked cereal food which can be prepared by merely admixing with water or milk.
It is well known to use a screw type extruder in which a cereal flour, admixed with water to adjust the water content to 12-30 percent by weight, is uniformly cooked and extruded as a continuous "rope" which is then sliced into pellets.
French Patent Application Publication No. 2 640 472 describes such a process in which a moistened cereal is cooked and extruded in an extrusion cooker.
On leaving the extruder die, the extruded product expands by a factor of about 3 and has a water content of about 12 to 20% by weight. This product is then sliced, partially dried, and ground in a mill with a 2.4 mm round hole screen.
The ground cereal product can then be dry mixed with supplementary thermosensitive ingredients.
The dry mixture is then granulated by any suitable agglomeration process, the resulting wet granular material being finally dried to a water content of about 4% to 7% by weight and packaged.
A ready-to-eat milk-dispersible cereal food is thus obtained.
The versatility of the extrusion-cooking process compared with drum drying is well known and the advantages of this process are for example avoidance of a large amount of floor space and of numerous operators. Nonetheless, extrusion cooking has several drawbacks.
First of all, since the product is intended to be a precooked cereal baby food, it is a major concern to be able to provide a dry product which, when mixed with a predetermined amount of milk, has a constant viscosity which, furthermore, is the same as the viscosity of cereal baby foods manufactured by drum drying.
On the other hand, it has been observed that extrusion cooking of cereals tends to push down the viscosity of the final product, and mothers, who are used to drum dried cereal baby foods, tend to lower the amount of added milk, which may lead to a nutritionally unbalanced product.
An object of the present invention is to provide an extrusion cooking process which, regardless of the formulation, leads to cereal baby foods having adequate viscosity when admixed with the proper amount of milk.
Moreover, extrusion cooking of cereals leads to a so-called "extrusion taste" which is an off-flavour released on admixing milk, and more particularly on admixing hot milk, and it is suspected that this "extrusion taste" is due to sulfur-containing compounds and other unknown compounds which are produced during extrusion-cooking probably because of the shearing of proteins and starch.
This problem is particularly acute in the manufacture of products where Maillard reactions do not occur, although Maillard reactions do not prevent the formation of sulfur-containing compounds but merely cover the "extrusion taste".
Another object of the present invention is to provide an extrusion cooking process which partly prevents the production of the extrusion taste.
Surprisingly, this taste is not observed with cereals obtained through drum drying. Such a difference could be explained by the fact that the solids content of the cereal slurry to be dried by conventional drum drying is always no greater than 50% by weight and usually greater than 25% by weight. It may be postulated that this high water content gives a very low viscosity when processing the base before drying and could prevent modifications of proteins and starch due to the absence of shearing.
The water content is much lower in extrusion cooking process in order to make it more energy efficient, the upper limit being a water content of the moist blend to be processed of no greater than 30% by weight. Furthermore, with such a high water content, the product would not puff at the die because it is too wet.
A further object of the present invention is to provide an extrusion-cooking process of the aforementioned type wherein the moist blend to be cooked has a solids content of at least 70% by weight.